Chromium tanning liquors



Oct. 24, 19

CHROMIUM TANNING LIQUORS Filed April 17, 1958 ACID TO BICHROMATE WEIGH RATIO |.2- l.6=l

CYCLOHEXANONE TO BICHROMATE (|.5=| PREFERRED) WEIGHT RATIO=.25=|

L L I \r H2O H2804 N0 Cr O -2H O CYCLOHEXANONE REACTOR r (30-50 c.) I I I l l I CRUDE FILTER ADIPIC ACID (0 c.) I l RECRYSTALLIZED I FROM 2N H 80 I l l MacrzoTHzo CHROMATE WEIGHT RATIO .9=l

k FILTER FILTRATE V SUGAR FI I E N T DUCE IS IEfi I REDUCED H2O CHROMIUM TO TRI- IF DESIRED DRIER CHRQME VALENT CHROMIUM LIQUOR 000 C.) ALUM v AS NECESSARY TO PURE ADIPIC ACID GIVE DESIRED CHARACTERISTICS TO TANNING LIQUOR CHROME SALT CAKE BASIC CHROME SULFATE TANNING LIQUOR INVENTOR I CLIFFORD N. MATTHEWS BYflM I cw ATTORNEYS United States Patent 3,005,678 CHRGMIUM TANNDIG LIQUORS Clifford N. Matthews, Painesville, Ohio, assignor to Diamend Alkali Company, Cleveland, Ohio, a corporation of Delaware Filed Apr. 17, 1958, Ser. No. 729,177 Claims. (Cl. 8--94.27)

This invention relates to a method for producing chromium tanning liquors and more particularly relates to a method for producing chromium tanning liquors wherein a useful by-product is produced in addition to the chro mium tanning liquor.

In the past, it has been the practice in preparing chromium tanning liquors, to react sulfuric acid anda source of hexavalent chromium, e.g., sodium bichromate, in a weight ratio of about '1 to 1, with a reducing agent, the purpose of the reducing agent being to reduce the hexavalent chromium to trivalent chromium. This reduced chrome liquor is then treated with various additives e.g., alum, organic and inorganic salts, etc., to obtain a tanning liquor having the desired tanning properties. Because of convenience and economy, the most common reducing agents which have been used are a sugar, e.g. tanning Sugar, or sulfur dioxide, which materials are oxidized mainly to gaseous products such as oxygen, hydrogen, carbon dioxide, and are readily expelled from the resulting reduced chrome liquor.

Although the above prior art process produces a chromium tanning liquor which satisfactorily fulfills the requirements of the tanning industry, it has been felt that the strong oxidizing action of the sodium bichromate or other hexavalcnt chromium compound in the above process is wasted in producing only a multiplicity of gases which are discarded. It was therefore believed that by using a different reducing agent than those commonly in use, a useful by-product could be produced by the oxidizing action of the sodium bichromate.

This problem cannot, however, be solved merely by using a reducing agent which is known to be oxidized to a useable product; but, rather, other factors must also be considered. Of primary importance, is the factor of whether the by-product formed is soluble in water or the reduced chrome liquor. If the product is water soluble, or soluble in the chrome liquor, then separation of the by-product and the chrome liquor will be very difficult, if not impossible, thus making the process economically unfeasible. Secondly, the by-product must not react with the chrome liquor or be decomposed thereby thus destroying the very result which is to be obtained. Thirdly, the by-product must be capable of being produced and recovered in relatively high yield. Finally, the economics of the by-product produced and the reducing agent used must be favorable, that is, the selling price of the byproduct must be above that of the reducing agent and this difference must be sufficiently great so as to compensate for any additional equipment and handling cost entailed in producing the by-product, so that its manufacture will be profitable.

Of the prior reducing agents which have been attempted to be used in such a process, e.g., toluene to give benzoic acid and isopropyl alcohol to give acetone, none have met all of the above requirements and hence their use has not been accepted.

It is therefore an object of the present invention to provide a method of producing chromium tanning liquors wherein a useful by-product is also produced.

Another object of this invention is to provide a process of the above type wherein the useful by-product produced is readily separable from the chromium tanning liquor, does not react therewith, and is produced in sufiiciently high yield to make the process economically feasible.

Patented Oct. 24, 1961 These and other objects will become apparent to those skilled in the art from the description of the invention which follows.

The drawing, which is attached hereto and made a part hereof, is a flow sheet showing a preferred embodiment of the method of the present invention and the manner in which it may be practiced.

It has now been found that by using a cyclohexyl compound as the reducing agent in the process for making chrome tanning liquors, in addition to the chrome tanning liquor, adipic acid is produced as a by-product of the reaction, which compound is produced in high yield and is readily separable from the chrome tanning liquor.

The method of the present invention envisions, generally, reducing an aqueous solution of an alkali metal bichromate in sulfuric acid, the sulfuric acid to alkali metal bichromate weight ratio being .9 to 1, using a cyclohexyl compound as the reducing agent, which compound is oxidized during the reaction to adipic acid. The adipic acid is separated from the reduced chrome liquor and purified, the reduced chrome liquor being further treated with a small amount of a sugar to insure the complete reduction of all the hexavalent chromium to trivalent chromium and with small amounts of various other additives to give the chromium tanning liquor the desired tanning properties.

Although the total amount of alkali metal bichromate may be added initially, it has been found that the highest yield of adipic acid is produced when the process is run as a two step process, that is, initially adding only sufficient alkali metal bichromate to react with all of the cyclohexyl compound and subsequently adding the remainder of the alkali metal bichromate after the byproduct adipic acid has been removed from the reduced chrome liquor. In view of the higher adipic acid yield produced, this latter method is therefore the preferred one.

By the term alkali metal bichromate, as used in the specification and claims, is meant the bichromates of sodium, potassium, lithium, cesium, rubidium, and francium. Of these, because of its low cost and availability, sodium bichromate is preferred, and hence in the description of the invention which follows, reference will be made primarily to sodium bichromate.

In the practice of the present invention, the cyclohexyl compounds have been found to be effective reducing agents for hexavalent chromium. Exemplary of these compounds are cyclohexane, cyclohexene, cyclohexanol, cyclohexanone and cyclohexyl oxide. Of these compounds, cyclohexanol and cyclohexanone have been found to be especially effective, with cyclohexanone being preferred.

In producting chromium tanning liquors by the preferred two step process of the present invention, an aqueous mixture of sodium bichromate and sulfuric acid having a weight ratio of from 1.2-1.6 to 1, acid to bichrw mate, 1.5 to 1 being preferred, is reduced with a cyclohexyl compound, the cyclohexyl to bichromate weight ratio being about .25 to 1. The temperature is maintained during the addition of the cyclohexyl compound, within the range of about 25-30" C. to about 50 C. It has been found that when higher temperatures are used, the yield of the by-product, adipic acid is reduced. Substantially all of the cyclohexyl compound is oxidized to adipic acid, which compound is separated from the reduced chrome liquor in any convenient manner, preferably by filtration, and then purified. To the reduced chrome found that the presence of these compounds is highly detrimental to the tanning action of the chromium tanning liquors. Suiiicient sugar is then added to the chrome liquor to reduce the hexavalent chromium of the second portion of sodium bichromate to trivalent chromium. The sugar is oxidized to gaseous products which pass out of the mixture. Sufiicient alum and chrome salt cake (sodium sulfate with about 1% sodium bichromate, a byproduct from the manufacture of sodium bichromate from sodium chromate liquors) are also added to the reduced chrome liquor so that the final product has a sufficient amount of sodium sulfate and aluminum oxide in its composition to give it the desired tanning qualities.

Referring now to the flow sheet drawing which illustrates one embodiment of. the preferred method of the present invention, sodium bichromate and sulfuric acid in the ratio of 1.5 acid to'l bichromate are added to the reactor and the hexavalent chromium of the sodium bichromate is reduced to trivalent chromium with cyclohexanone, the cyclohexanone to bichromate weight ratio being about .25 to 1. The reactor is maintained within the temperature range of 30 to 50 C. during the reaction. When the reaction is complete, the reaction mixture is filtered at C., a crude adipic acid being retained in the filter and the reduced chrome liquor forming the filtrate. The crude adipic acid is then purified by recrystallization from a weak sulfuric acid solution, e.-g., 2 N sulfuric acid, filtered and dried to produce pure adipic acid. To the reduced chrome liquor which is the filtrate, a sugar in aqueous solution, alum, chrome salt cake and sufiicient bichromate to make the total acid to sodium bichromate ratio .9 to l is added. This mixture is maintained at 100 C. until all of the hexavalent chromium in the sodium bichromate which has been reduced to trivalent chromium. The solution produced by this reaction is the basic chrome sulfate tanning liquor.

In adding the cyclohexyl compound to the sulfuric acid sodium bichromate mixture, it is desirable that the rate of addition be such that the cyclohexyl compound is oxidized as soon as it comes in contact with the bichromate. An' addition time for the cyclohexyl compound of about 1 to 5 hours has been found to be satisfactory, with 4 hours being the preferred time. 7 When the cyclohexyl compound is added at this rate, the heat of the exothermic reduction reaction will maintain the reaction temperature within the desired range, i.e., 30 to 50 C., with little or no external. cooling being necessary.

Following the separation of crude adipic acid from the reduced chrome liquor, the crude adipic acid appears as a green, particulate mass containing adipic acid with occluded chromium compounds thereon. The adipic acid may be conveniently purified by recrystallizing it from a solution ofa dilute acid, e.g., 2 N sulfuric acid, although other methods or recrystallizing solvents may be used. The purified adipic acid is separated from the as to reduce all the hexavalent chromium, in the second portion of sodium bichromate which is added, to trivalent chromium. Finally, sufficient alum and chrome salt cake are added to give the resulting chrome tanning liquor the desired tanning characteristics. The temperature of the reduced chrome liquor during these additions is preferably maintained at about 100 C. At this temperature, it has been found that complete reduction of all the hexavalent chromium occurs most rapidly with little adverse eifect on the equipment used.

In order that those skilled in the art may better understand the method of-the present invention and the manner in which it may be practiced, the following specific examples are offered.

Example I 7,350 g. of sulfuric acid (96%) 4,900 g. of sodium bichromate and 15,000 g. of Water are mixed in a lead tank which is fitted with lead cooling coils. To the mixture is added with constant stirring over a period of 3 /2 hours, 1,300 mls. of cyclohexanone. The weight ratio of the acid to the bichromate in the mixture is 1.5 to 1 and the weight ratio of the cyclohexanone to the bichromate is .26 to 1. The temperature of the mixture is maintained at 25 C. When all the cyclohexanone is reacted with the sodium bichromate, the adipic acid formed is separated from the reduced chromium liquor by filtration. The adipic acid is purified by recrystallization from 2 N sulfuric acid. The yield of adipic acid is 1438 g. representing a percent yield of 79% based on a the amount of cyclohexanone used.

acid solution by filtration and dried, the product thus obtained being in marketable condition. The filtrate from the purification is found to contain some dissolved adipic acid, chromium compounds, and dilute sulfuric acid. If desired this filtrate can be recycled into the reactor, thereby making up a portion of the sulfuric acid charge for the next reaction. The components of the filtrate are thereby recovered, thus making the over-all process more efficient.

In the preferred two step process the product of the second step is the basic chrome sulfate tanning liquor. To obtain this product, to the filtrate of the crude adipic acid product of the first step, is added sufficient sodium bichromate to make the total acid to bichromate weight ratio .9 to 1. The use of this weight ratio gives a chrome tanning liquor having the desired basicity and chrome content. Moreover, the second addition'of sodium bichromate, insuresthat all of the cyclohexyl compound has been oxidized. Sufficient sugar is also added, so

To the reduced chrome liquor filtrate which, has been separated from the adipic acid is added 3267 g. of sodium bichromate, 1091 g. of tanning sugar, 800 g. of chrome salt cake, and 1633 g. of alum, with constant stirring, While maintaining the temperature of the mixture at C. When all of the sodium bichromate has been reduced by the tanning sugar, as evidenced by the cessation of the evolution of gases from the mixture, the resulting mixture is the basic chrome sulfate tanning liquor.

Inasmuch as the tanning liquor produced by this processv Willbe the same regardless of what the initial acid to bichromate ratio is, the process differing only in the amount of reactants added, in the remaining examples, only the first step of the process will be shown, i.e., the step resulting in the production of the adipic acid.

Example 11 Using the same apparatus as in Example I, 4900 g. of sodium bichromate, 2346 mls. of sulfuric acid (96%), 500 mls. of water, and 10,000 mls. of the recrystallization liquor from a previous adipic acid purification, containing 400 mls. of sulfuric acid (96%), are mixed together. The acid to bichrornate weight ratio is .9 to 1. 1300 mls. of cyclohexanone is added slowly over a period of 4 hours while maintaining the temperature between 45 -50 C. with constant agitation. The mixture is stirred for 1 an additional hour to insure the complete oxidation of all of the cyclohexanone. The adipic acid formed is'separated from the reduced chrome liquor and purified by recrystallization from an aqueous solution of 2 N sulfuric acid. 968 g. of pure adipic acid are formed representing a yield of 54% based on the amount of cyclohexanone used.

Example III Using the same apparatus as in Example I, 1300 mls. of cyclohexanone is added slowly over a period of 3% hours to a mixture of 5,000 g. of sodium bichromate, 3350 mls. of sulfuric acid (96%) and 11,000 mls. of water. The reaction mixture has an acid to bichromate weight ratio of 1.03 to 1. The temperature is maintained at 25 C. during the addition of the cyclohexanone. The mixture is stirred following the addition of the cyclohexanonefor a period of about 1 hour to insure complete oxidation of the cyclohexanone. The adipic acid formed is separated from the reduced chrome liquor and purified by recrystallization from a 2 N sulfuric acid solution. 1127 g. of adipic acid, representing a yield of 62% based on the amount of cyclohexanone used are formed.

Example IV Using the same apparatus as in Example I, 1300 mls. of cyclohexanone is added over a period of 3% hours to a mixture of 5000 g. of sodium bichromate, 6350 g. of sulfuric acid (96%) and 11,000 mls. of water, with constant stirring. The mixture is maintained at a temperature of 25 C. during the addition of the cyclohexanone. The acid to sodium bichromate ratio is 1.27 to 1. Following the addition of the cyclohexanone, the mixture is stirred for an additional hour to insure the complete oxidation of the cyclohexanone. The resulting adipic acid which is formed is separated from the chrome liquor by filtration and purified by recrystallization with an aqueous solution of 2 N sulfuric acid. 1280 g. of pure adipic acid are formed, representing a 70% yield based on the amount of cyclohexanone used.

Example V Using a one-liter, three-necked flask fitted with a stirrer, a spiral condenser, a dropping funnel and a thermometer, 49 g. of cyclohexanone are added with constant stirring to a mixture of 140 mls. of 96% sulfuric acid, 200 g. of sodium bichromate and 460 mls. of water. The temperature of the reaction mixture is maintained at 40 C. The acid to bichromate weight ratio is 1.30 to 1. Following the addition of the cyclohexanone, the reaction mixture is stirred for an additional 1.5 hours to insure the complete oxidation to the cyclohexanone. 55 g. of crude adipic acid are formed representing a yield of 75% crude adipic acid based on the amount of cyclohexanone used.

Example VI Using the same apparatus as in Example I, 1300 mls. of cyclohexanone are added over a period of 4 hours, to a mixture containing 4900 g. of sodium bichromate, 7153 g. of 96% sulfuric acid, 500 mls. of water, and 10,000 mls. of recrystallization liquor from a previous adipic acid purification, containing 400 mls. of 96% sulfuric acid. The reaction mixture has an acid to bichromate weight ratio of 1.61 to 1. During the addition of the cyclohexanone the temperature of the mixture is maintained between 4550 C. The mixture is stirred for an additional hour after the addition of the cyclohexanone is complete to insure the complete oxidation of the cyclohexanone. The crude adipic acid formed is separated by filtration from the reduced chrome liquor and purified by recrystallization from an aqueous solution of 2 N sulfuric acid. 1480 g. of pure adipic acid, representing a 77% yield based on the amount of cyclohexanone used, are formed.

Example VII Using the same apparatus as in Example I, 1300 mls. of cyclohexanone are added with constant stirring over a period of 4 hours to a mixture containing 4900 g. of sodium bichromate, 7594 g. of 96% sulfuric acid, 5000 mls. of water, and 10,000 mls. of recrystallization liquor, from a previous purification of adipic acid, containing 400 mls. of 96% sulfuric acid. The temperature of the mixture is maintained between 45-50 C. The reaction mixture has an acid to bichromate weight ratio of 1.70 to 1. Following the addition of the cyclohexanone the mixture is stirred for an additional hour to insure the complete oxidation of the cyclohexanone. The crude adipic acid formed is separated by filtration from the reduced chromium liquor and purified by recrystallization from an aqueous solution of 2 N sulfuric acid. 1398 g. of adipic acid are formed representing a 76% yield based on the amount of cyclohexanone used.

6 Example VIII Using the same apparatus as in Example V, 32.8 g. of cyclohexene are added to a mixture containing 134 mls. of 96% sulfuric acid, 250 g. of sodium bichromate, and 454 mls. of water over a period of 3 hours. The reaction mixture has an acid to bichromate weight ratio of .99 to l. The temperature is maintained within the range of 3050 C. during the addition of the cyclohexene. Following the addition of the cyclohexene the mixture is stirred for an additional 2 hours to insure the complete oxidation of the cyclohexene. 25.5 g. of crude adipic acid are formed representing a yield of 62% crude adipic acid based on the amount of cyclohexene used.

Example IX Using the same apparatus as in Example V, 20 g. of cyclohexanol are added to a mixture containing 134 mls. of 96% sulfuric acid, .250 g. of sodium bichromate and 454 mls. of water, over a period of 2 hours. The mixtime has an acid to bichromate weight ratio of .99 to 1. During the addition of the cyclohexanol, the temperature of the mixture is maintained at 30 C. by constant agitation. The mixture is stirred for an additional one and one-half hours after the addition of the cyclohexanol to insure its complete oxidation. 20.5 g. of crude adipic acid are formed representing a yield of 69% crude adipic acid based on the amount of cyclohexanol used.

Example X A sample of the purified adipic acid from Example I is analyzed and compared to a sample of commercial adipic acid available on the market. Results of this comparison are shown below:

' Purified Commercial Theoretical Adipic Acid Sample of Value from Ex- Adipic Acid ample I Melting Point, C 151-153 151-151. 5 151-151. 5 Neutralization Equivalent. 73 72 71.1 Percent Carbon 49. 32 49. 24 49.18 Percent Hydrogen 6. 6. 67 6. 58

From the above table it can be seen that the adipic acid produced by the method of the present invention is equivalent to that available commercially at the present time.

Example XI A sample of the basic chrome sulfate tanning liquor prepared in Example I is compared to a sample of a commercially available basic chrome sulfate tanning liquor. Results of this comparison are shown below:

From the above it can be seen that the basic chrome sulfate tanning liquor produced by the method of the present invention is comparable in composition to that which is commercially available.

As the above examples show, that although various cyclohexyl compounds may be used in the method of the present invention with varying acid to bichromate weight ratios, the highest yields based on the amount of the cycloh-exyl compound used, are obtained when using cyclohexanone in the two step process with an acid to 7 bichromate weight ratio of abouf 1.5 to 1, in the first reaction step. Hence, this is the preferred method.

Additionally, it is apparent that by the method of the present invention, both a basic chrome sulfate tanning liquor and a useful .by-product, i.e., adipic acid, are produced. These products are comparable in composition and purity to those'now presently available on the market. Moreover, it has been found, that by using a cyclohexyl compound as the reducing agent, excluding the profit realized from the sale ofthe lay-product, ,adipic acid, the cost of producing the basic chrome sulfate 'tanning liquor is reduced.

While there have been described in detail certain forms of the invention and embodiments of its practice, the invention is not to be understood as being limited to the detailed disclosure as .it is realized that changes within the scope of the invention are possible, and it is further intended that each step in the following claims shall refer to all equivalent steps for accomplishing the same result in substantially the same or equivalent manner, it being intended to cover this invention broadly in whatever form its principle may be realized.

' What is claimed is: i

i 1. A method of producinga chromium tanning liquor whereby a useful 'by-product is also produced which comprises reacting an alkali metal bichromate'and a cyclohexyl compound in the presence of sulfuric acid, and separating the by-product, adipic acid, thus formed from the chromium tanning liquor. v a

2. A method of making a basic chrome sulfate tanning liquor whereby a useful by-product is also produced which comprises reacting an alkali metal bichromate and a cyclohexyl compound in the presence of sulfuric acid, separating the-'adipic acid' by-product-thereby formed from the reduced chrome liquor; adding an additional quantity of the alkali metal bichromate and a sugar, thereby forming the basic chrome'sul-fate tanning liquor.

3. The method' as claimed'in claim-2 'wherein the alkali metal bichromate "is sodium bichromate;

4. The method as claimed in 'clairn 3 wherein the cyclohexyl compound is cyclohexanone. H

5. The method as claimed inclaim 3 wherein the cyclohexyl compound is cyclohexanol.

6. The method as claimed in claim 3 wherein the cyclohexyl compound is cyclohexene- 7. The method as claimed in claim 3 wherein the cyclohexyl compound is cyclohexane.

8. The method of making a basic chrome sulfate tanning liquor whereby adipic acid is also produced as a useful by-product, comprising reacting 1 part by Weight of sodium bic'hromate with .25 part byweight of a cycloheiiylcompound'in the presenceof 19 part by weight sulfuric acid and sufficient sugar, alum, and chromesalt cake to completely reduce all hexavalent chromium to trivalent chromium and" give theresulting tanning liquor the desired tanning properties, and separating the adipic acid thus formed from the basic chrome sulfate tanning liquor;

.9. Themeth-od as claimed in claim 8 wherein the cyclohexyl compound is cyclohexan'one.

10. The method as claimed in claim'8 wherein the cyclohexyl compound is cyclohexanol.

.11. The method as claimed in claim 8 wherein the cyclohexyl compound is cyclohexene.

12. The method of making a basic chrome sulfate tanning liquor whereby the adipic acid is also formed as a useful byproduct, comprising reacting 1 part by Weight of sodium bichromate with .25 part by weight of a cyclohexyl compound in the presence of.l.2 to 1.6 parts by weight sulfuric acid, separating the adipic acid thus formedfrom the reduced chrome liquor, adding an additional quantity of sodium bichromate to the reduced chrome liquor in amount sufiicient to make the over-all weight ratio of sulfuric acid to sodium bichromate .9 to 1, and adding suiiicient sugar, alum, and chrome salt cake-to the liquor' to completely reduce all of the hexavalent chromium to trivalent chromium and give the resulting basic chrome sulfate tanning liquor the'desired tanning'properties.

. 13. The method as claimed in claim 12 wherein the cyclohexyl compound is cyclohexanone.

. 14. The method as claimed in claim 12 wherein the cyclohexyl compound is cyclohexanol;

15. The method as claimed in claim 12 wherein the cycloh'exyl compound is cyclohexene.

1 6.'The method as claimed in claim 12 wherein the cyclohexyl compound is cyclohexane.

17. The method as claimed in claim 12 wherein the amount of sulfuric acid in the original reaction is 1.5 parts by weight.

18. The method as claimed in claim 17 wherein the cyclohexyl compound is cyclohexanol.

19, The method as claimed in claim 17 wherein the cyclohexyl compound is cyclohexene.

' 20. The method as claimed in claim 17 wherein the cyclohexyl compound-is cyclohexane.

References Cited in theme of this patent UNITED STATES PATENTS 

1. A METHOD OF PRODUCING A CHROMIUM TANNING LIQUOR WHEREBY A USEFUL BY-PRODUCT IS ALOS PRODUCED WHICH COMPRISES REACTING AN ALKALI METAL BICHROMATE AND A CYCLOHEXYL COMPOUND IN THE PRESENCE OF SULFURIC ACID, AND SEPARATING THE BY-PRODUCT, ADIPIC ACID, THUS FORMED FROM THE CHROMIUM TANNING LIQUOR. 